Transducers, or sensors, for transmitting and receiving ultrasonic energy in applications involving liquids are well known. A typical application for such transducers is in liquid level sensing to sense the height of a liquid in a tube or pipe or a container or other vessel, such as a tank. The term vessel as used hereinafter includes any type of a vessel such as a tank or other liquid container and the term tube includes pipes of metal or plastic and tubing of flexible or rigid material. Another application is use with a tube to transmit and receive ultrasonic energy across a defined space so as to sense the presence of air bubble or particles flowing in the tube so that they can be counted and characterized, such as by size, by a computer. Still a further application is in flow meters where the rate of flow in a pipe is measured.
One widely used form of such transducer has a housing that contains a piezo-electric element. The housing is mounted to the vessel or tube with the front end of the housing extending trough a hole placed to be in direct contact with the liquid. The ultrasonic energy is coupled directly to the liquid to sense its presence or the energy dissipates in air when no liquid is present.
Another type of ultrasonic transducer is of the non-contact type, such as disclosed in U.S. Pat. No. 4,630,245, granted Dec. 16, 1986. Here, the transducer housing is of a rigid material, such as a plastic, and is fixedly mounted, such as by a clamp or strap, to the outside of the vessel or tube in which the liquid being sensed is present. The non-contact type transducer has an advantage in that no hole has to be made in the vessel or tube to which it is mounted but it requires a coupling compound, such as Vaseline, or silicon grease, between the housing and the vessel or tube. The coupling compound is needed so that there will be no air gaps between the end of the transducer housing through which ultrasonic energy enters or is transmitted and the vessel or tube since this would adversely affect the transmission and reception of the ultrasonic energy from the transducer to the wall of the vessel or tube and thereafter to the liquid or air therein.
Application of the coupling compound in the foregoing type of non-contact transducer is another step required in the mounting of the transducer. The need for using a coupling compound also limits the use of such a transducer, for example, in an application for sensing the liquid level in bottles moving rapidly past a fixed inspection point on an assembly line. Here, it would not be possible to apply the coupling compound to each of the bottles. Also, in some applications, the use of a coupling compound can adversely affect the appearance of the tube or vessel. Further, in sensitive sanitary applications such as drug and food processing, the use of such coupling compound should or must be avoided. Another disadvantage in this type of non-contact transducer is that it is fixed in position and cannot be easily moved.
U.S. Pat. No. 6,781,287 to Naim Dam, et al, granted Aug. 24, 2004, which is assigned to the assignee of the subject application discloses an ultrasonic transducer that has a capsule, or head, of a flexible and deformable material that contacts the outer surface of the pipe or vessel. The piezoelectric element is within the capsule which also contains a fluid, such as oil, that serves as the coupling agent. While this transducer does not require a coupling agent external of the transducer housing, it is somewhat complicated and relatively expensive to manufacture.
Accordingly, a need exists for a non-contact type ultrasonic transducer that is not subject to the limitations of requiring either an externally applied coupling compound or external mounting devices. It is also preferred that such a transducer be as economical as possible so that it can be disposable when used in certain applications, such as medical procedures.